The invention relates to a protection relay comprising a processing unit connected to means for measuring supply signals representative of heat rises caused by the flow of currents in electrical conductors, and means connected to the processing units for setting and supplying values representative of protection thresholds.
Known protection relays, such as electronic trip devices for circuit breakers, enable electrical contacts to be opened in order to protect an installation. The trip devices generally comprise current sensors connected to a processing unit which performs protection functions. The processing unit makes a trip relay operate when a signal supplied by the current sensors exceeds a threshold for a preset duration.
Certain trip devices comprise temperature sensors connected to the processing unit. These sensors supply a temperature signal to enable a current to be interrupted when an excessive heat rise occurs in the circuit breaker.
Temperature sensors can also be used to determine the thermal memory of an installation. A device of this kind, described in European Patent EP 0 421 892 B, proposes simulation of cooling after the circuit breaker contacts have opened.
Long delay tripping functions can use temperature signals but the tripping thresholds are not very precise and the time delay setting possibilities are fairly limited. Therefore trip devices often combine a very precise protection by measuring current and a complementary or redundant protection by measuring temperature.
Furthermore, known tripping functions use temperature signals which do not allow a short delay protection.